1. Field of the Invention
The present invention relates to a filter circuit and a communication device.
2. Description of the Related Art
The power supply voltage of a radio frequency (RF) circuit tends to decrease along with miniaturization of a complementary metal oxide semiconductor (CMOS) process. Due to the miniaturization of the CMOS process, if an RF circuit is mounted using a known circuit technique, a voltage margin becomes insufficient and the dynamic range of the signal amplitude is narrowed. Meanwhile, the cut-off frequency of a transistor increases due to the miniaturization of the CMOS process, which is advantageous for operations in which high-speed switching operations are performed accurately at a precise timing. In addition, it is also advantageous that the capacitor ratio becomes accurate due to highly accurate lithography.
A digital RF technology is a new technology that introduces the concept of discrete time signal processing to the RF circuit, in order to avoid the problems that may be caused as a result of the miniaturization of the CMOS process and to obtain the advantages. A charge domain filter is used as a main circuit in the digital RF technology field. The charge domain filter is a filter circuit that is formed by a transconductance amplifier, switches and capacitors. The charge domain filter performs sampling of analog signals by accumulating and discharging electric charges in synchronization with a clock, and performs filtering, decimation and the like using discrete time signal processing.
As a known example of the charge domain filter, there is a charge domain second order infinite impulse response low pass filter (IIR-LPF) such as that described in S. Manetti and A. Liberatore, “Switched-capacitor lowpass filter without active components,” Electron. Lett., 1980, 16, pp. 883-885. The charge domain second order IIR-LPF is advantageous in that it can be realized by a simple charge domain circuit.